Corvette Transmission Build - Slip Slidin' Away, Part II

Making The 700-R4 Better, Stronger, And Faster

In last month's issue, we tore into The Blubonic Plague's very tired 700-R4 and showed the catastrophic effects of a bad Throttle Valve (TV) cable. Actually, it was a cheap 10-cent plastic clip that went bad. The TV cable comes from the throttle valve in the transmission and connects to the throttle body on the engine. This cable tells the transmission the position of the throttle. The transmission increases or decreases line pressure according to throttle position. Without this connection, the trans would never raise the line pressure, and the internals would burn up because they couldn't handle the load. Now, we're going to follow Mike Bastio of Toy Shop Transmissions as he re-incarnates The Plague's stricken tranny into a formidable piece of automotive machinery.

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The 700-R4 has been refined and upgraded over the years, and having rebuilt a few thousand of these babies in his career, Mike knows what works and what doesn't. He can tailor each transmission to the needs of the customer. Our needs were easy to address: we wanted a street trans that shifted firmly, delivered good mileage, and could handle a ton of horsepower (eventually). Armed with that information, Mike proceeded to get down to business. He noticed during the teardown that the transmission had already been rebuilt once and had a couple of upgrades. One of the upgrades already present was the replacement of the low/reverse reaction sun gear with a stronger unit. Other areas that Mike concentrates on (by modifications to the valve body) are fluid flow, pressure regulation, and improved shifting. When you think about it, most of Mike's modifications are based on common sense, but without the intimate knowledge of the inner workings of the transmission, they're useless.

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The finished product was, in a word, impressive. The Blubonic Plague has a whole new attitude, and although I know I'm supposed to get better gas mileage, I haven't, because I'm having too much fun sticking my foot in the throttle to get the most out of every tire-barking shift! Once I settle down, I expect my combined mileage to be in the 15-16 mpg range. Before we go any further, I must say that I made a mistake in part one of this article. I referred to a part that I called the 3-4 drum, when it's actually the reverse input drum. The band that holds it is called the 2-4 band, meaning it holds the drum in second and fourth gear. Now that I've gotten my terminology correct, let's get started.

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Corvette Transmission Build - Slip Slidin' Away, Part II

The first thing Mike does is reassemble each clutch pack with new clutches and new steel plates. That includes the overrun clutches, forward clutches, and the 3-4 clutches. He added an extra clutch in the 3-4 section for improved clutch grip.

The toasted reverse input drum was replaced with a new one. Here, it's placed on the input housing before it's set aside.

Pictured here is the rear planetary gear set. In the center (arrow) is the low/reverse sun gear. In earlier transmissions, this piece broke on a routine basis. Later transmissions feature an upgraded unit that can be retrofitted to earlier units. Ours already had the upgraded gear, so it was good enough to keep. The planetary gears have been exchanged for newer units with improved thrust washers. This gear set, along with the front planetary gear set, goes in first.

Another improvement is the low roller clutch. You can easily see that the one on the right is a little taller and stronger. There's another one-way clutch in the input housing that is also upgraded.

Next, the input components are placed inside the transmission housing.

The 2-4 band is the next component to go in. The band grips and holds the reverse input drum during second and fourth gear. The weak spot is where the band is anchored. Hard shifts will cause the anchor to snap.

In this picture, the old anchor is on the left. You can see where the new anchor on the right is substantially beefier than its predecessor. It's much more capable of handling firm shifts.

Mike then turned his attention to the front pump. A common problem is that the front seal blows out prematurely. He alleviates this ailment by drilling out the fluid passage directly behind the front seal to relieve the pressure.

Another weak point on the front pump is that the bearing will spin in its bore. Mike dimples the bearing with a hammer and punch to keep it in its place.

Another improvement GM made over the years was to increase the number of vanes in the pump. At the bottom right is the original seven-vane rotor from The Plague. Next to it is the upgraded 10-vane rotor, with the 13-vane unit on top. The additional vanes keep the line pressure stable and prevent the fluid from pulsing. Mike installed the 13-vane rotor.

Although it's more of an annoyance than a problem, some 700-R4s would buzz when put into reverse. To alleviate this, many rebuilders would take the pressure regulator valve out of the pump and cut down this ridge (see pointer). Unfortunately, this also reduces line pressure inside the entire transmission, which can lead to burned clutches and other damage. Mike replaces this component with a new one.

Before the seal goes on, Mike gives it a shot of Loctite to make sure it stays put.

The front pump is secured to the transmission case using new bolts with new O-ring seals-they're much better than the old washer-type seals.

Here, Mike pulls out the check ball from the input shaft. This will help the torque converter to lock up more firmly.

When we came to the accumulators, Mike gave me an option. "I can leave the springs in for soft shifts, or I can replace the springs with these spacers for hard shifts." Needless to say, I went with the spacers.

Mike opened up a couple of choice holes in the valve body plate. This also lends itself towards firmer shifts.

To improve the part-throttle shifts, Mike replaces the detent spring with a firmer one (the spring on the right). The firmer spring keeps the transmission from shifting too soon. This will aid in performance as well as mileage.

Now for some electronic work. On the valve body, there is a one-terminal pressure switch. Although it's a pressure switch, the computer controls it. Under certain conditions, the computer will ground the switch, locking up the torque converter (think of it as the mandatory 1-4 shift on late-model GM six-speed cars). Mike exchanges the single terminal switch for a dual terminal switch and wires it accordingly. This way, the switch is controlled exclusively by pressure, and locks up the converter in fourth gear only.

Modifications complete, Mike reassembles the rest of the transmission.

After Mike gives the transmission his trademark black paint job, he installs the torque converter. The converter's stall speed is raised to 2,200 rpm-great for the street.

Beto Diaz re-installs the fresh tranny and bolts it in place.

With the trans in and the car on the ground, Mike installs a new TV cable. Once it's filled with fluid, we're ready for some street poundin'!

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